The use of specialized ink formulations to form thick films having various functions on suitable substrates in the construction of multilayer integrated circuits is known in the art. Such technology is of increasing interest in the production of very dense multilayer circuit patterns on various substrates for a wide variety of applications in the electronics industry.
Thick film multilayer structures typically are comprised of at least two patterned layers of a conductor separated by a dielectric layer. The patterned conductor layers are connected by metallic conductor deposited in vias in the dielectric layer. Such structures are formed by multiple deposition and firing of layers of conductor and dielectric inks.
Such multilayer circuit structures utilizing copper as the conductor metal have a number of problems. The most common is failure caused by the development of electrical shorts due to penetration of the dielectric layers by flux components of the copper conductor ink, such as copper and bismuth oxides, which takes place during the multiple firings necessary to produce a multilayer circuit structure. When a conductive channel formed by the penetration of flux components passes completely through a dielectric layer and makes contact between an overlying and underlying copper conductor, an electrical short is produced.
A second problem common to multilayer circuits is porosity in the fired dielectric layers resulting from the evolution of gases from organic vehicle materils and/or oxides of bismuth and copper during firing. Contaminant materials e.g., molten eutectic phases from fired copper conductor layers, can readily leach into the resulting passages. The integrity of the dielectric layers in a multilayer structure is also important because the voids therein lower the resistance of the dielectric which is undesirable. It is conventional to print and fire at least two layers of dielectric ink between conductive layers to minimize the possibility that evolving gases will form connected passages through the dielectric.
It is possible to reduce the porosity of a dielectric material by formulating the ink with a higher quantity of glass frit. However, both this solution and the multiple print and fire approach can result in the trapping of gases within the dielectric layer. This will cause both the dielectric layer and overlying conductor layers to blister and peel. Added to these considerations is the fact that a vitreous glass can alter its viscosity in response to reactions with external flux phases, such as copper and bismuth oxides. Therefore, it becomes very difficult to predict whether a given glass/filler mixture will yield an impenetrable dielectric for multilayer applications. Such an improved dielectric is, however, provided in accordance with this invention.